An air-cooled turbine blade according to the invention includes a cavity within which an insert of variable size is provided. As the insert varies in size, the size of a cooling passage at least partially defined by the insert is also changed. Such turbine blades are used as guide or moving blades, for example, in gas turbines.
It has been known to cool the parts of thermal turbomachines subjected to hot gas, such as, for example, turbine blades of gas turbines, by means of a cooling medium in order to be able to increase the temperature of the hot gas, and to prolong the service life of the parts concerned by reducing thermal loading. For this purpose, cooling holes are provided in the wall of the turbine blade and other cooling systems are provided in the interior of the turbine blade. The inside of the turbine blade is cooled as the cooling air dissipates the heat to the outside. The outside of the blade is cooled by a film which forms on the surface of the turbine blade.
In the case of conventional turbine blades, the cavity may be provided with one or more inserts in order to obtain a certain cooling structure. Such inserts are described, for example, in publications DE-A-23 20 581, EP 0 534 207, EP 0 182 588 or also in DE-A1476 790. In these disclosures, there are in some cases, in addition to the insert, further built-in components in the interior in order to cool certain parts of the turbine blade in an improved manner relative to other parts or to utilize the cooling air more effectively. Thus there are ribs on the wall of the cavity, which together with the insert form clearly defined cooling passages, constrictions for improved cooling, for example of the pressure side of the blade, or cooling holes in the insert for the (impingement) cooling of the leading edge or of one of the two sides.
A turbine blade has also been disclosed by U.S. Pat. No. 4,859,141, which provides an insert which is made of a shape memory alloy. When a certain temperature threshold value is exceeded, the insert expands and in this way bears free of play against the internal geometry.
For the development of turbine blades, which must meet the continuously increasing temperatures of gas turbines, it is essential to provide improved cooling systems. Conventional inserts, however, do not allow for variances in the size of the cooling system, and in particular the size of the cooling passages, as a function of the temperature of the hot gases. This is because conventional inserts are rigid and the size of the cooling system/cooling passages does not change as a function of the temperature during the operation of the gas turbine. This has an adverse effect, since it is desirable to cool certain regions of the turbine blade more intensively relative to other regions of the turbine blade as a function of the temperature acting from outside.
The invention avoids the above mentioned disadvantages. An air-cooled turbine blade includes an insert which regulates the cooling (effectiveness of the cooling, cooling air quantity, size of cooling passages) in the interior of the cavity as a function of the temperature of the hot gas during operation of the gas turbine.
A turbine blade includes an insert, wherein the size of the insert, after a temperature predetermined by the composition of the shape memory alloy is exceeded, is changed in such a way that the cooling system is changed, i.e. enlarged or reduced in size, as a result.
In order to reduce the overall size of the insert, it is especially advantageous if the insert includes a strap with two overlapping ends. In a thermally activated state, the insert can be reduced in size overall by the two ends being pushed one over the other.
The insert can also be formed with only individual portions of the insert being provided with a shape memory alloy. This allows the insert to be used to specifically cool certain parts of the turbine blade in an intensified manner. One or more articulations in the form of bends or curves in the shape memory alloy can be provided to permit local changes in the size of the insert. This may be effected, for example, in the vicinity of the leading edge of the turbine blade, on the pressure or suction side, or over the height of the blade body.
The insert can also be used in connection with ribs arranged radially or axially on the inner wall of the cavity. In this case, the insert can be shaped to cover the ribs and reduce the size of the cooling passages defined between the ribs by portions that project into the intermediate spaces between the ribs. When the insert contracts as a result of a change in temperature, the ribs are covered in a flush manner and the cooling passages are enlarged as the portions withdraw from between the ribs.
In order to utilize the effect of the contraction of the insert in the thermally activated state, and also to ensure retention of the insert on the blade body of the turbine blade, it is necessary to mechanically fasten the insert by brazing or welding to one or more points on the wall of the blade body, to the ribs, to the tip, or to the platform of the turbine blade.
Cooling holes may also be advantageously provided in the insert in order to intensify the effect of the cooling.
Shape memory alloys used for the insert can include, for example, NiTi, Cuxe2x80x94Znxe2x80x94Al or Cuxe2x80x94Alxe2x80x94Ni.